Securing a fresnel lens to a refractive optical element

ABSTRACT

A headset for virtual reality applications includes an optical element configured to modify light from an electronic display in the headset and to direct the modified light to a user. The optical element may include a Fresnel lens secured to a lens by securing the Fresnel lens to a mold and inserting a casting material into the mold so the casting material forms the lens and a portion of the casting material exists on and past an edge of the Fresnel lens. This encases the edge of the Fresnel lens in the casting material, securing the Fresnel lens to the lens.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/742,886, filed on Jun. 18, 2015, which is hereby incorporated byreference in its entirety.

BACKGROUND

This disclosure relates generally to manufacturing processes, and morespecifically to securing a Fresnel lens to a refractive optical element.

Electronic displays include a plurality of pixels, which may eachinclude a plurality of sub-pixels (e.g., a red sub-pixel, a greensub-pixel, etc.). Arrangement of individual sub-pixels may affect theappearance and performance of an electronic display device. Somearrangements of sub-pixels may increase fixed pattern noise undercertain conditions. For example, magnification of a pixel may result inboundaries between individual sub-pixels of the pixel becoming visibleto the user, resulting in a “screen door” pattern (i.e., an increase infixed pattern noise) in an image presented to a user. While correctiveoptical elements may be used to reduce the effect of fixed pattern noisein content presented by the user, conventional corrective opticalelements are difficult to rapidly manufacture. For example, certaintypes of corrective optical elements are lenses in which multiplegrooves are etched, which precludes the lenses from being molded. Theadditional time and expense of etching grooves into the lenses after thelenses are molded increases the time and expense in producing thesecorrective optical elements.

SUMMARY

An optical element for viewing content presented via an electronicdisplay includes a diffractive element, such as a Fresnel lens, thatmodifies light presented by the electronic display and directs themodified light to a user for presentation. For example, the Fresnel lensblurs light generated by different sub-pixels in an image presented bythe electronic display to prevent the user from seeing dark spacebetween the sub-pixels and a refractive optical element (e.g., a lens)directs the blurred light to a user's eye. The Fresnel lens includes aseries of equally spaced grooves, with the distance between the groovesreferred to as “pitch width.” The pitch width determines the amount bywhich light from sub-pixels presented by the electronic display isblurred by the Fresnel lens. However, the grooves included in a Fresnellens prevent the Fresnel lens from being fabricated via molding.Instead, a lens is initially generated via a molding process, and thegrooves are subsequently etched into the lens to produce the Fresnellens, which increases the time and complexity of generating the Fresnellens.

To simplify production of the optical element for viewing contentpresented via the electronic display, a Fresnel lens or other suitablediffractive element is secured to a surface of a molding structure. Forexample, the Fresnel lens is secured to a surface of a molding structurethrough one or more pins inserted through an exterior portion of theFresnel lens (e.g., a portion within a threshold distance of an edge ofthe Fresnel lens and outside of a field of view of a user) and into thesurface of the molding structure. The exterior portion of the Fresnellens may be an edge of the surface of Fresnel lens or a portion of thesurface of Fresnel lens between a specified distance from the edge ofthe surface of Fresnel lens and the edge of the surface of the Fresnellens (e.g., from the edge of the Fresnel lens to 0.1 millimeters fromthe edge of the Fresnel lens). In various embodiments, the exteriorportion of the surface of the Fresnel lens is specified so that it isoutside of a field of view of a user who views data through the Fresnellens or through an optical element coupled to the Fresnel lens. Aftersecuring the Fresnel lens to the surface of the molding structure, themold is assembled using one or more additional portions. For example, anadditional portion of the molding structure is positioned relative tothe portion of the molding structure to which the Fresnel lens issecured. In various embodiments, the additional portion of the moldingstructure is positioned so there is a specified distance between asurface of the Fresnel lens and an inner surface of the additionalportion of the molding structure. Distances between different locationson the inner surface of the additional portion of the molding structureand a location on the surface of the Fresnel lens may differ in someembodiments, so different locations on the inner surface of theadditional portion of the molding structure have different distances tothe location on the surface of the Fresnel lens.

In some embodiments, when the mold is assembled, the assembled mold alsohas a specified distance between the surface of the Fresnel lens and aninner surface of the portion of the molding structure to which theFresnel lens is secured. For example, the specified distance between thesurfaces of the Fresnel lens and the inner surface of the portion of themolding structure is along an exterior portion of the surface of Fresnellens from the surface of the Fresnel lens to the inner surface of theportion of the molding structure. The assembled mold may also have adistance between an exterior portion of an additional surface of theFresnel lens that is parallel to the surface of the Fresnel lens (e.g.,a surface of the Fresnel lens nearer to the molding structure) and theinner surface of the portion of the molding structure to which theFresnel lens is secured, creating separation between the additionalsurface of the Fresnel lens and the inner surface of the moldingstructure between the edge of the Fresnel lens and a location on theadditional surface of the Fresnel lens that is a specified distance fromthe edge of the Fresnel lens. Additionally, the exterior portion of thesurface of Fresnel lens may include one or more openings extending fromthe surface of the Fresnel lens through the thickness of the Fresnellens or through a portion of the thickness of the Fresnel lens.

A casting material, such as resin, that is transmissible to one or morewavelengths of light is inserted into the assembled mold, forming alayer between the additional portion of the molding structure and thesurface of the Fresnel lens that has a thickness equaling the specifieddistance between the surface of the Fresnel lens and the inner surfaceof the additional portion of the molding structure. In some embodiments,the layer formed between the surface of the Fresnel lens and the innersurface of the additional portion of the molding structure creates alens or other refractive element that affects the focusing of lightpassing through the layer. Distances between a location on the surfaceof the Fresnel lens and locations on the inner surface of the additionalportion of the molding structure determine the curvature of the lens invarious embodiments. If the assembled mold has a specified distancebetween the surface of the Fresnel lens and an inner surface of theportion of the molding structure to which the Fresnel lens is secured,inserting the casting material into the assembled mold forms a layer ofthe casting material between the surface of the Fresnel lens and theportion of the molding structure. For example, if the specified distanceis along an exterior portion of the Fresnel lens from the surface of theFresnel lens to the inner surface of the portion of the moldingstructure, a layer of the casting material is formed along the exteriorportion of the Fresnel lens from the surface of the Fresnel lens to theinner surface of the portion of the molding structure. This encases theexterior portion of the Fresnel lens in the casting material from thesurface of the Fresnel lens to the inner surface of the portion of themolding structure. In embodiments where the assembled mold has adistance between an exterior portion of an additional surface of theFresnel lens that is parallel to the surface of the Fresnel lens (e.g.,a surface of the Fresnel lens nearer to the molding structure) and theinner surface of the portion of the molding structure to which theFresnel lens is secured, inserting the casting material into theassembled mold also generates a layer of casting material between theadditional surface of the Fresnel lens and the inner surface of themolding structure. This layer of casting material secures the Fresnellens to the layer of casting material between the surface of the Fresnellens and the inner surface of the additional molding structure. Hence,the casting material forms a layer between the additional surface of theFresnel lens and the inner surface of the molding structure extending anoverlap distance from the edge of the Fresnel lens to a location on theadditional surface of the Fresnel lens. In embodiments where the Fresnellens includes one or more openings in the exterior portion of thesurface of the Fresnel lens, inserting the casting material into theassembled mold causes the casting material to flow through the openings,forming molded pins. The assembled mold is subsequently removed afterthe casting material cures or hardens to produce an optical elementwhere the Fresnel lens is secured to a refractive optical element, suchas a lens, that directs light from the Fresnel lens to a user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a wire diagram of a virtual reality (VR) headset, inaccordance with an embodiment.

FIG. 1B is a cross section of a front rigid body of the VR headset inFIG. 1A, in accordance with an embodiment.

FIG. 2 is an optical block of a VR headset including a Fresnel lens anda refractive optical element, in accordance with an embodiment.

FIG. 3 is a flowchart of a method for securing a Fresnel lens to arefractive optical element, according to one embodiment.

FIG. 4A is an example of securing a Fresnel lens to a mold, according toone embodiment.

FIG. 4B is an additional example of securing a Fresnel lens to a mold,according to one embodiment

FIG. 4C is an example of inserting a casting material into a mold towhich a Fresnel lens has been secured, according to one embodiment.

FIG. 4D is an example of a Fresnel lens secured to a refractive opticalmaterial generated by casting material, according to one embodiment.

The figures depict various embodiments for purposes of illustrationonly. One skilled in the art will readily recognize from the followingdiscussion that alternative embodiments of the structures and methodsillustrated herein may be employed without departing from the principlesdescribed herein.

DETAILED DESCRIPTION Example Outer Shell

FIG. 1A is a wire diagram of a virtual reality (VR) headset, inaccordance with an embodiment. The VR headset 100 includes a front rigidbody 105 and a band 110. The front rigid body 105 includes one or moreelectronic display elements of an electronic display, and may include aninertial measurement unit (IMU) 130, one or more position sensors 117,and locators 125. In the embodiment shown by FIG. 1A, the positionsensors 117 are located within the IMU 130, and neither the IMU 130 northe position sensors 117 are visible to the user.

The locators 125 are located in fixed positions on the front rigid body105 relative to one another and relative to a reference point 115. Inthe example of FIG. 1A, the reference point 115 is located at the centerof the IMU 130. Each of the locators 125 emits light that is detectableby an imaging device. Locators 125, or portions of locators 125, arelocated on a front side 120A, a top side 120B, a bottom side 120C, aright side 120D, and a left side 120E of the front rigid body 105 in theexample of FIG. 1A.

The IMU 130 is an electronic device that generates fast calibration databased on measurement signals received from one or more of the positionsensors 117. A position sensor 117 generates one or more measurementsignals in response to motion of the VR headset 100. Examples ofposition sensors 117 include: one or more accelerometers, one or moregyroscopes, one or more magnetometers, another suitable type of sensorthat detects motion, a type of sensor used for error correction of theIMU 130, or some combination thereof. The position sensors 117 may belocated external to the IMU 130, internal to the IMU 130, or somecombination thereof.

Based on the one or more measurement signals from one or more positionsensors 117, the IMU 130 generates fast calibration data indicating anestimated position of the VR headset 100 relative to an initial positionof the VR headset 100. For example, the position sensors 117 includemultiple accelerometers to measure translational motion (forward/back,up/down, left/right) and multiple gyroscopes to measure rotationalmotion (e.g., pitch, yaw, roll). In some embodiments, the IMU 130rapidly samples the measurement signals and calculates the estimatedposition of the VR headset 100 from the sampled data. For example, theIMU 130 integrates the measurement signals received from theaccelerometers over time to estimate a velocity vector and integratesthe velocity vector over time to determine an estimated position of areference point on the VR headset 100. Alternatively, the IMU 130provides the sampled measurement signals to the VR console 110, whichdetermines the fast calibration data. The reference point is a pointthat may be used to describe the position of the VR headset 100, such asthe reference point 115. While the reference point 115 may generally bedefined as a point in space, the reference point 115 is defined as apoint within the VR headset 100 (e.g., a center of the IMU 130) invarious embodiments.

The IMU 130 receives one or more calibration parameters from a virtualreality (VR) console and uses the one or more calibration parameters tomaintain tracking of the VR headset 100. Based on a received calibrationparameter, the IMU 130 may adjust one or more IMU parameters (e.g.,sample rate). In some embodiments, certain calibration parameters causethe IMU 130 to update an initial position of the reference point so itcorresponds to a next calibrated position of the reference point.Updating the initial position of the reference point as the nextcalibrated position of the reference point helps reduce accumulatederror associated with the determined estimated position. The accumulatederror, also referred to as drift error, causes the estimated position ofthe reference point to “drift” away from the actual position of thereference point over time.

FIG. 1B is a cross section 180 of the front rigid body 105 of theembodiment of a VR headset 100 shown in FIG. 1A. As shown in FIG. 1B,the front rigid body 105 includes an optical block 140, which providesaltered image light to an exit pupil 150. The exit pupil 150 is thelocation of the front rigid body 105 where a user's eye 135 ispositioned. For purposes of illustration, FIG. 1B shows a cross section180 associated with a single eye 135, but another optical block,separate from the optical block 140, provides altered image light toanother eye of the user.

The optical block 140 includes an electronic display element 145 of anelectronic display that projects image light toward the correctiveoptics block 118, which is included in the optical block 140 and altersthe projected image. For example, the corrective optics block 118magnifies and corrects optical errors associated with the projectedimage. The optical block 140 is configured to correct for fixed patternnoise by slightly blurring sub-pixels. The corrective optics block 118directs the altered image light to the exit pupil 150 for presentationto the user.

The electronic display element 145 includes a display area comprising aplurality of sub-pixels, where a sub-pixel is a discrete light emittingcomponent. Different sub-pixels are separated from each other by darkspace. For example, a sub-pixel emits red light, yellow light, bluelight, green light, white light, or any other suitable color of light.In some embodiments, images projected by the electronic display element145 are rendered on the sub-pixel level. This is distinct from, say anRGB (red-green-blue) layout, which has discrete red, green, and bluepixels (red, green, and blue) and each pixel in the RGB layout includesa red sub-pixel, which is adjacent to a green sub-pixel that is adjacentto a blue sub-pixel; the red, green, and blue sub-pixels operatetogether to form different colors. In an RGB layout a sub-pixel in apixel is restricted to working within that pixel. However, in someembodiments, sub-pixels in the electronic display element 145 operatewithin multiple “logical” pixels in their surrounding vicinity to formdifferent colors. The sub-pixels are arranged on the display area of theelectronic display element 145 in a sub-pixel array. Examples of asub-pixel array include PENTILE® RGBG, PENTILE® RGBW, some anothersuitable arrangement of sub-pixels that renders images at the sub-pixellevel.

The corrective optics block 118 includes one or more optical elementsthat adjust an image projected by the electronic display element 145 tothe user by the VR headset 100. In some embodiments, the correctiveoptics block 118 is positioned at least 35 mm from the electronicdisplay element 145. At least a portion of an optical element in thecorrective optics block 118 includes a diffractive surface. In variousembodiments, an optical element in the corrective optics block 118includes a refractive surface (e.g., a concave surface), a diffractivesurface (e.g., a Fresnel surface, a binary surface, some other type ofdiffractive element), or some combination thereof. Portions of thediffractive surface and/or the refractive surface may include a flatportion, a curved portion, or both. The diffractive surface of anoptical element may be uniform or may have a higher density of groovesnear the center of the optical element. A diffractive optical element isan optical element including at least a portion of a diffractivesurface. Additionally, in some embodiments, an optical element may be anaperture, a filter, or any other suitable optical element that affectsthe image projected by the electronic display element 145. In someembodiments, one or more of the optical elements in the correctiveoptics block 118 may have one or more coatings, such as anti-reflectivecoatings.

The corrective optics block 118 magnifies image light projected by theelectronic display element 145 and corrects optical errors associatedwith the image light. Magnification of the image light allows theelectronic display element 145 to be physically smaller, weigh less, andconsume less power than larger displays. Additionally, magnification mayincrease a field of view of the displayed media. For example, the fieldof view of the displayed media is such that the displayed media ispresented using almost all (e.g., 110 degrees diagonal), and in somecases all, of the user's field of view. However, magnification may causean increase in fixed pattern noise, also referred to as the “screen dooreffect,” which is a visual artifact where dark space separating pixelsand/or sub-pixels of a display become visible to a user in an imagepresented by the display. For example, magnification without opticalerror correction may increase fixed pattern noise to the point where theprojected image suffers from the screen door effect. In someembodiments, the corrective optics block 118 is designed so itseffective focal length is larger than the spacing to the electronicdisplay element 145, which magnifies the image light projected by theelectronic display element 145. Additionally, in some embodiments, theamount of magnification may be adjusted by adding or removing opticalelements.

The corrective optics block 118 may be designed to correct one or moretypes of optical error. Optical error may be fixed pattern noise (i.e.,the screen door effect), two dimensional optical errors, threedimensional optical errors, or some combination thereof. Two dimensionalerrors are optical aberrations that occur in two dimensions. Exampletypes of two dimensional errors include: barrel distortion, pincushiondistortion, longitudinal chromatic aberration, transverse chromaticaberration, or any other type of two-dimensional optical error. Threedimensional errors are optical errors that occur in three dimensions.Example types of three dimensional errors include spherical aberration,chromatic aberration, field curvature, astigmatism, or any other type ofthree-dimensional optical error. The corrective optics block 118 maycorrect for fixed pattern noise by slightly blurring the image of eachsub-pixel so the blurred sub-pixels mask the dark space between thesub-pixels via a Fresnel lens or other diffractive surface. In someembodiments, the media provided to the electronic display element 145for display is pre-distorted, and the corrective optics block 118corrects the distortion.

FIG. 2 is an example optical block 200 where the corrective optics block118 includes an optical element 210 having a diffractive surface, suchas a Fresnel surface 220, and a separate refractive optical element 230(e.g., a lens). The optical element 210 and the separate refractiveoptical element 230 are shaped and positioned to magnify the electronicdisplay element 145 and correct for fixed pattern noise, as well ascorrect for one or more additional optical errors. Because the opticalblock 200 shown in FIG. 2 has discrete refractive and diffractiveelements, the optical block 200 is simpler to manufacture than anoptical block combining the diffractive and refractive properties into asingle optical element, which may result in difficulties inmanufacturing and potential problems with glare. The Fresnel surface 220of the optical element 210 is positioned to receive image light from theelectronic display element 145 and generate blur spots, by diffractingimage light from the electronic display element 145.

The refractive optical element 230 is a convex lens that provides thediffracted image light to an exit pupil 150. As shown in FIG. 2, therefractive optical element 230 includes a first surface 232 thatreceives diffracted light from the optical element 210 and a secondsurface 234 that directs the diffracted light toward an exit pupil 150.The first surface 232 and the second surface 234 have differentcurvatures, with the curvatures of the surfaces 232, 234 selected todirect the diffracted light to the exit pupil 150, to minimizeintroduction of optical error, to correct one or more optical errors, orto perform any suitable function.

Process for Securing a Fresnel Lens to a Lens

FIG. 3 is one embodiment of a method for securing a Fresnel lens, orother diffractive surface, to a lens or other refractive opticalelement. In various embodiments, the method may include different and/oradditional steps than those described in conjunction with FIG. 3.Additionally, in some embodiments, steps of the method may be performedin different orders.

Initially, a diffractive element, such as a Fresnel lens, is secured 310to a surface of a molding structure. For example, the Fresnel lens issecured to a surface of a molding structure through one or more pinsinserted through an exterior portion of the Fresnel lens (e.g., aportion within a threshold distance of an edge of the Fresnel lens) andinto the surface of the molding structure. Alternatively, the Fresnellens is secured 310 to a surface of the molding structure throughsuction (e.g., through creating a vacuum). For example, the surface ofthe molding structure includes one or more openings, allowing a pressuredifference between the surface of the molding structure and anothersurface of the molding structure parallel to the surface to secure 310the Fresnel lens to the surface of the molding structure (e.g., throughcreating a vacuum).

After securing 310 the Fresnel lens to the surface of the moldingstructure, the mold is assembled 320 using one or more additionalportions. For example, an additional portion of the molding structure ispositioned relative to the portion of the molding structure to which theFresnel lens is secured 310. In various embodiments, the additionalportion of the molding structure is positioned so there is a specifieddistance between a surface of the Fresnel lens and an inner surface ofthe additional portion of the molding structure. For example, thespecified distance is between a center of the surface of the Fresnellens and a specific location of the inner surface of the additionalportion of the molding structure. In some embodiments, there aredifferent distances between different locations on the surface of theFresnel lens and different locations on the inner surface of theadditional portion of the molding structure. Alternatively, theadditional portion of the molding structure is positioned so a distancebetween the center of the surface of the Fresnel lens and variouslocations on the inner surface of the additional portion of the moldingstructure is constant (e.g., positioned so a semicircle with a specificradius from the center of the surface from the Fresnel lens is formedbetween the surface of the Fresnel lens and the inner surface of theadditional portion of the molding structure).

In some embodiments, when the mold is assembled, the assembled mold hasa specified distance between the surface of the Fresnel lens and aninner surface of the portion of the molding structure to which theFresnel lens is secured 310. For example, the specified distance isalong an exterior portion of the surface of Fresnel lens from thesurface of the Fresnel lens to the inner surface of the portion of themolding structure. In some embodiments, the specified distance betweenthe surface of the Fresnel lens and the inner surface of the portion ofthe molding structure to which the Fresnel lens is secured 310 is equalto the thickness of the Fresnel lens. Alternatively, the specifieddistance between the surface of the Fresnel lens and the inner surfaceof the portion of the molding structure to which the Fresnel lens issecured 310 is equal to the thickness of the Fresnel lens incremented bya value, so the specified distance is greater than the thickness of theFresnel lens. The exterior portion of the Fresnel lens may be an edge ofthe surface of Fresnel lens or a portion of the surface of Fresnel lensbetween a specified distance from the edge of the surface of Fresnellens and the edge of the surface of the Fresnel lens (e.g., from theedge of the Fresnel lens to 0.1 millimeters from the edge of the Fresnellens). In various embodiments, the exterior portion of the surface ofthe Fresnel lens is specified so that it is outside of a field of viewof a user who views data through the Fresnel lens or through an opticalelement coupled to the Fresnel lens.

The assembled mold may have a distance between the inner surface of theportion of the molding structure to which the Fresnel lens is securedand an exterior portion of an additional surface of the Fresnel lensthat is parallel to the surface of the Fresnel lens (e.g., a surface ofthe Fresnel lens nearer to the portion of the molding structure). Hence,there is separation between the additional surface of the Fresnel lensand the inner surface of the molding structure between the edge of theFresnel lens and a location on the additional surface of the Fresnellens that is a specified distance from the edge of the Fresnel lens,also referred to as an “overlap distance.” Additionally, the exteriorportion of the surface of Fresnel lens may include one or more openingsextending from the surface of the Fresnel lens through the thickness ofthe Fresnel lens or through a portion of the thickness of the Fresnellens.

A casting material, such as resin, that is transmissible to one or morewavelengths of light is inserted 330 into the assembled mold. Thecasting material forms a layer between the additional portion of themolding structure and the surface of the Fresnel lens that has athickness equaling the specified distance between the surface of theFresnel lens and the inner surface of the additional portion of themolding structure. In some embodiments, the layer formed between thesurface of the Fresnel lens and the inner surface of the additionalportion of the molding structure creates a lens that affects thefocusing of light passing through the layer. Distances between alocation on the surface of the Fresnel lens and locations on the innersurface of the additional portion of the molding structure determine thecurvature of the lens in various embodiments. Additionally, if theassembled mold has a specified distance between the surface of theFresnel lens and an inner surface of the portion of the moldingstructure to which the Fresnel lens is secured 310, inserting 330 thecasting material into the assembled mold forms a layer of the castingmaterial between the surface of the Fresnel lens and the portion of themolding structure. For example, if the specified distance between thesurface of the Fresnel lens and an inner surface of the portion of themolding structure to which the Fresnel lens is secured 310 is along anexterior portion of the Fresnel lens from the surface of the Fresnellens to the inner surface of the portion of the molding structure, alayer of the casting material is formed along the exterior portion ofthe Fresnel lens from the surface of the Fresnel lens to the innersurface of the portion of the molding structure. This configurationencases the exterior portion of the Fresnel lens in the casting materialfrom the surface of the Fresnel lens to the inner surface of the portionof the molding structure. In some embodiments, the assembled mold has adistance between an exterior portion of an additional surface of theFresnel lens that is parallel to the surface of the Fresnel lens (e.g.,a surface of the Fresnel lens nearer to the molding structure) and theinner surface of the portion of the molding structure to which theFresnel lens is secured, so inserting 330 the casting material into theassembled mold also generates a layer of casting material between theadditional surface of the Fresnel lens and the inner surface of themolding structure. Hence, the casting material forms a layer between theadditional surface of the Fresnel lens and the inner surface of themolding structure extending an overlap distance from the edge of theFresnel lens to a location on the additional surface of the Fresnellens. If the Fresnel lens includes one or more openings in the exteriorportion of the surface of the Fresnel lens, inserting 330 the castingmaterial into the assembled mold causes the casting material to flowthrough the openings, which forms molded pins when the casting materialcures. The assembled mold is subsequently removed 340 after the castingmaterial cures or hardens to produce an optical element where theFresnel lens is secured to a lens.

FIG. 4A is an example of an assembled mold 400 with a Fresnel lens 220secured to a surface of a portion of a molding structure 410A. Theassembled mold 400 includes an additional molding structure 420 having asurface that is separated from a surface of the Fresnel lens 220 by afirst specified distance 430. In the example of FIG. 4A, variouslocations along the surface of the additional molding structure 420 havea common distance from a location in the center of the surface of theFresnel lens 220. Additionally, in the example of FIG. 4A, the assembledmold 400 has a second specified distance 440 between the surface of theFresnel lens 220 and an inner surface of the portion of the moldingstructure 410A. For example, the second specified distance 440 is alongan exterior portion of the surface of Fresnel lens 220 from the surfaceof the Fresnel lens 220 to the inner surface of the portion of themolding structure 410A. As described above in conjunction with FIG. 3,the exterior portion of the Fresnel lens 220 may be an edge of thesurface of Fresnel lens 220 or a portion of the surface of Fresnel lens220 between a specified distance from the edge of the surface of Fresnellens 220 and the edge of the surface of the Fresnel lens 220 that isoutside of a field of view of a user who views data through the Fresnellens or through an optical element coupled to the Fresnel lens 220.

FIG. 4B is an additional example of the assembled mold 400 with aFresnel lens 220 secured to a surface secured to a surface of a portionof a molding structure 410B. As in the example of FIG. 4A, the assembledmold 400 includes an additional molding structure 420 having a surfacethat is separated from a surface of the Fresnel lens 220 by a firstspecified distance 430 and has a second specified distance 440 betweenthe surface of the Fresnel lens 220 and an inner surface of the portionof the molding structure 410B. Additionally, the assembled mold 400 inFIG. 4B have a distance between the inner surface of the portion of themolding structure 410B and an exterior portion of an additional surfaceof the Fresnel lens 220 extending from the edge of the Fresnel lens 220an overlap distance 450 into the additional surface of the Fresnel lens220. This creates separation between the additional surface of theFresnel lens 220 and the inner surface of the portion of the moldingstructure 410B along the overlap distance 450 between the edge of theFresnel lens and a location on the additional surface of the Fresnellens 220 that is a specified distance from the edge of the Fresnel lens220.

FIG. 4C is an example of inserting a casting material into an assembledmold 400 to which a Fresnel lens has been secured, according to oneembodiment. In FIG. 4C, a casting material, such as resin, is insertedinto the assembled mold 400 shown in FIG. 4A. The casting material fillsthe first specified distance 430 between the surface of the Fresnel lens220 and the surface of the additional portion of the molding structure420 as well as the second specified distance 440 between the surface ofthe Fresnel lens 220 and the inner surface of the portion of the moldingstructure 410A. In other embodiments, inserting the casting materialinto the assembled mold 400 fills distances between the inner surface ofthe portion of the molding structure to which the Fresnel lens 220 issecured and an additional surface of the Fresnel lens 220, such asdistances between the additional surface of the Fresnel lens 220 and theinner surface of a molding structure 410B along the overlap distance 450shown in FIG. 4B. After the casting material has set, cured, orhardened, FIG. 4D shows an optical element 450 including the Fresnellens 220 secured to a refractive optical element 230 formed by the curedcasting material.

Securing the Fresnel lens to a portion of a surface of a moldingstructure and subsequently inserting casting material into an assembledmold inclosing the molding structure and the Fresnel lens allows theFresnel lens to be secured to a refractive optical element, such as alens, produced when the casting material cures. Separation betweenregions of the Fresnel lens and the surface of the molding structureallows the casting material to encase portions of the Fresnel lens(e.g., an edge of the Fresnel lens, an amount of the Fresnel lensbetween the edge and a specified distance from the edge), which securesthe Fresnel lens to the refractive optical element generated whencasting material between a surface of the Fresnel lens and a surface ofa portion of an additional molding structure cures. While the precedingexamples describe securing a Fresnel lens to a refractive opticalelement, in other embodiments, any suitable diffractive optical elementmay be secured to the portion of the surface of the molding structureand casting material inserted into an assembled mold including thediffractive optical element and the molding structure, as describedabove in conjunction with FIGS. 2-4D.

SUMMARY

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the invention be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative, but not limiting, ofthe scope of the invention, which is set forth in the following claims.

What is claimed is:
 1. A virtual reality headset comprising: an opticalblock with one end adjacent to an exit pupil corresponding to a positionof a user's eye when using the virtual reality headset, the opticalblock comprising: an electronic display element opposite the exit pupil,the electronic display element projecting image light toward the exitpupil; a corrective optics block between the electronic display and theexit pupil, the corrective optics block comprising a diffractive opticalelement and a refractive optical element, the corrective optics blockconfigured to correct one or more types of optical error in theprojected image light.
 2. The virtual reality headset of claim 1,wherein the diffractive optical element is a Fresnel lens.
 3. Thevirtual reality headset of claim 1, wherein the diffractive opticalelement has a higher density of grooves near its center.
 4. The virtualreality headset of claim 1, wherein the refractive optical element is aconvex lens.
 5. The virtual reality headset of claim 1, wherein the oneor more types of optical error comprise fixed pattern noise.
 6. Thevirtual reality headset of claim 5, wherein the corrective optics blockis configured to slightly blur image light projected from sub-pixels ofthe electronic display element.
 7. The virtual reality headset of claim1, wherein the diffractive optical element is secured to the refractiveoptical element via molded pins.
 8. The virtual reality headset of claim7, wherein the molded pins are formed through an exterior portion of thediffractive optical element that is located outside of a field of viewof the user when their eye is positioned at the exit pupil.
 9. Thevirtual reality headset of claim 1, wherein the corrective optical blockis positioned at least 35 millimeters from the electronic displayelement.
 10. The virtual reality headset of claim 1, wherein aneffective focal length of the corrective optics block is larger than adistance from the corrective optics block to the electronic displayelement.
 11. The virtual reality headset of claim 1, wherein thecorrective optical block comprises a resin.
 12. A virtual realityheadset comprising: a first optical block with one end adjacent to afirst exit pupil corresponding to a position of a user's first eye whenusing the virtual reality headset, the optical block comprising: a firstelectronic display element opposite the first exit pupil, the firstelectronic display element projecting image light toward the first exitpupil; and a first corrective optics block between the first electronicdisplay and the first exit pupil, the first corrective optics blockcomprising a first diffractive optical element and configured to correctone or more types of optical error in the projected image light; and asecond optical block with one end adjacent to a second exit pupilcorresponding to a position of the user's second eye when using thevirtual reality headset, the second optical block separate from thefirst optical block and comprising: a second electronic display elementopposite the second exit pupil, the second electronic display elementprojecting image light toward the second exit pupil; and a secondcorrective optics block between the second electronic display and thesecond exit pupil, the second corrective optics block comprising asecond diffractive optical element and configured to correct the one ormore types of optical error in the projected image light.
 13. Thevirtual reality headset of claim 12, wherein the first and seconddiffractive optical elements are Fresnel lenses.
 14. The virtual realityheadset of claim 12, wherein each of the first and second diffractiveoptical elements has a higher density of grooves near its center. 15.The virtual reality headset of claim 12, wherein each of the first andsecond corrective optics blocks further comprise a refractive opticalelement.
 16. The virtual reality headset of claim 15, wherein therefractive optical element is a convex lens.
 17. The virtual realityheadset of claim 15, wherein each of the first and second diffractiveoptical elements is secured to the corresponding refractive opticalelement via molded pins.
 18. The virtual reality headset of claim 12,wherein the one or more types of optical error comprise fixed patternnoise.
 19. The virtual reality headset of claim 17, wherein each of thefirst and second corrective optics block are configured to slightly blurimage light projected from sub-pixels of the corresponding electronicdisplay element.
 20. The virtual reality headset of claim 12, whereinfor each of the first and second corrective optics blocks, an effectivefocal length of the corrective optics block is larger than a distancefrom the corrective optics block to the corresponding electronic displayelement.